字符串相等(==)的全测试

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本文通过一个Java示例程序详细解析了字符串在不同情况下使用“==”进行比较的行为。包括相同包内不同类间的字符串常量引用、不同包内的字符串常量引用以及运行时计算生成的字符串之间的比较。

 package p1;
/**
 * 字符串相等(==)的全测试。
 * 
 * @author XT Zang,老紫竹
 */
class TestString1 {
  public static void main(String[] args) {
    String hello = "Hello", lo = "lo";
    System.out.print((hello == "Hello") + " "); // true
    System.out.print((Other.hello == hello) + " "); // true
    System.out.print((p2.Other.hello == hello) + " "); // true
    System.out.print((hello == ("Hel" + "lo")) + " "); // true
    System.out.print((hello == ("Hel" + lo)) + " "); // false
    System.out.println(hello == ("Hel" + lo).intern()); // true
  }
}
class Other {
  static String hello = "Hello";
}

 package p2;
public class Other {
  public static String hello = "Hello";
}

结论：

This example illustrates six points:

Literal strings within the same class (§8) in the same package (§7) represent references to the same String object (§4.3.1).相同包，相同类的字符串常量指向相同的字符串对象
Literal strings within different classes in the same package represent references to the same String object.相同包，不相同类的字符串常量指向相同的字符串对象
Literal strings within different classes in different packages likewise represent references to the same String object.不同包，不同类的字符串常量指向相同的字符串对象
Strings computed by constant expressions (§15.28) are computed at compile time and then treated as if they were literals.字符串在编译期间可以连接起来的，视同一个常量字符串
Strings computed by concatenation at run time are newly created and therefore distinct.